tor-browser

commit 7267ee8b9a4f4632607186ff33f711ba671ae5b1
parent bac754a046f857fada39f1a12adf3c8a7627a1d8
Author: Jens Stutte <jstutte@mozilla.com>
Date:   Mon,  3 Nov 2025 14:08:41 +0000

Bug 1997924 - Avoid memory allocations in ProcessUpdatedPriorityModifier. r=bas

We can leverage std::set's extract / insert to avoid allocations when updating task priorities.
It is assumed that this is always better, also for a small number of tasks.

Differential Revision: https://phabricator.services.mozilla.com/D269142

Diffstat:
M
xpcom/threads/TaskController.cpp
 | 
32
++++++++++++++------------------
M
xpcom/threads/TaskController.h
 | 
5
+++--

2 files changed, 17 insertions(+), 20 deletions(-)
diff --git a/xpcom/threads/TaskController.cpp b/xpcom/threads/TaskController.cpp
@@ -1494,26 +1494,22 @@ void TaskController::ProcessUpdatedPriorityModifier(TaskManager* aManager) {
 
   int32_t modifier = aManager->mCurrentPriorityModifier;
 
-  std::vector<RefPtr<Task>> storedTasks;
-  // Find all relevant tasks.
-  for (auto iter = mMainThreadTasks.begin(); iter != mMainThreadTasks.end();) {
-    if ((*iter)->mTaskManager == aManager) {
-      storedTasks.push_back(*iter);
-      iter = mMainThreadTasks.erase(iter);
-    } else {
-      iter++;
+  // Find all relevant task nodes and move them to a temporary set with the
+  // new priority modifier.
+  PrioritySortedTasks managerTasks;
+  auto cur = mMainThreadTasks.begin();
+  while (cur != mMainThreadTasks.end()) {
+    // Keep a valid iterator before potentially extracting the current task.
+    auto next = std::next(cur);
+    if (cur->get()->mTaskManager == aManager) {
+      auto task = mMainThreadTasks.extract(cur);
+      task.value()->mPriorityModifier = modifier;
+      managerTasks.insert(std::move(task));
     }
+    cur = std::move(next);
   }
-
-  // Reinsert found tasks with their new priorities.
-  for (RefPtr<Task>& ref : storedTasks) {
-    // Kept alive at first by the vector and then by mMainThreadTasks.
-    Task* task = ref;
-    task->mPriorityModifier = modifier;
-    auto insertion = mMainThreadTasks.insert(std::move(ref));
-    MOZ_ASSERT(insertion.second);
-    task->mIterator = insertion.first;
-  }
+  // Merge the temporary set back to the main set.
+  mMainThreadTasks.merge(std::move(managerTasks));
 }
 
 }  // namespace mozilla
diff --git a/xpcom/threads/TaskController.h b/xpcom/threads/TaskController.h
@@ -431,8 +431,9 @@ class TaskController {
   std::stack<RefPtr<Task>> mCurrentTasksMT;
 
   // A list of all tasks ordered by priority.
-  std::set<RefPtr<Task>, Task::PriorityCompare> mThreadableTasks;
-  std::set<RefPtr<Task>, Task::PriorityCompare> mMainThreadTasks;
+  using PrioritySortedTasks = std::set<RefPtr<Task>, Task::PriorityCompare>;
+  PrioritySortedTasks mThreadableTasks;
+  PrioritySortedTasks mMainThreadTasks;
 
   // TaskManagers currently active.
   // We can use a raw pointer since tasks always hold on to their TaskManager.